Edmonds ADU Permit Requirements, in Plain English

The short answer

Can I Build an ADU in Edmonds?

An ADU is an accessory dwelling unit. It is a second, smaller home on the same lot as your house. It can be attached, like a basement apartment or an addition. Or it can be detached, like a backyard cottage.

LDR-L is Edmonds' Low Density Residential, Large Lot zone. It covers most of the single family lots in the city. On these lots, you can build up to two ADUs. Each one can be up to 1,200 square feet. You do not have to live on the property. Your first ADU does not need a new parking space.

That is more room than most homeowners expect. Most Edmonds projects do not get stopped by the rules on paper. They get slowed down or reshaped by the land itself and by the work it takes to make a space safe and legal to live in. This page walks through both.

First, Make Sure Edmonds Is Your Permit City

Your mailing address and your permit city are not always the same. Some homes with an Edmonds address sit outside city limits, in unincorporated Snohomish County, which has its own rules. Before you plan anything, look up your parcel on the Edmonds Web Map. If the city holds your parcel, the rules on this page apply to you.

Confirmed rules

Edmonds ADU Rules: What Is Confirmed

Zoning and How Many Units You Can Build

Edmonds allows up to two ADUs on a lot. You can mix them any way you want. Two attached, two detached, or one of each. An attached ADU can be built from space you already have, like a basement. That is often the lower cost path compared to building a new detached unit.

Size Limits

Each ADU can be up to 1,200 square feet. That is enough for a two bedroom, two bath home. Lots we have reviewed, from roughly 10,800 to 19,400 square feet, cleared Edmonds' rules with no minimum lot size problem.

Height Limits

A detached ADU can be up to 24 feet tall. That leaves room for two stories. An attached ADU follows the same 25 foot limit as your main house.

Setbacks

A setback is the minimum distance between a building and your property line. In Edmonds, any new ADU must sit at least 25 feet from the street and at least 10 feet from each side line. This is true for attached and detached units.

The rear setback is 25 feet. There is one important exception. If a detached ADU sits next to a public alley, it can be built right at that alley line with no setback at all. The alley has to be a true public alley that touches your property line. A private drive or an unused strip does not count. This one detail can move a backyard cottage 25 feet, so it is worth confirming early.

Owner Occupancy

Edmonds does not require you to live on the property. You do not have to live on site to build or rent an ADU.

Parking

Your first ADU needs no new parking space. A second ADU needs one parking space.

Entrance and Utilities

Every ADU needs its own entrance, separate from the main house. Edmonds allows one water meter and one sewer line for the whole lot, so your ADU can share them with your house. Any new utility lines must go underground.

Fees

The transportation impact fee for an ADU is $4,306.83, effective July 1, 2026. An impact fee is a one time charge that helps the city pay for roads and growth. The ADU rate is half of what a new house pays.

You will also pay sewer, water and stormwater charges, set at current city rates. Building permit and plan review fees come from the city's fee guide. All of these are posted on the city's Development Fees page. Get a current number from the city before you set your budget, since these change.

How You Apply

You apply online through MyBuildingPermit.com. The city's ADU handout asks for a site plan, floor plans, a count of plumbing fixtures, and the locations of smoke alarms. If your lot has slopes or adds a lot of hard surface, soil and stormwater documents may be added to that list.

Do I Need to Hire an Architect?

No, not by law. Washington state does not require a licensed architect for homes with one to four units, and an ADU falls under that rule. Edmonds does not add an architect requirement of its own. A homeowner, a designer or a design build firm can prepare the drawings. You can read the state rule yourself at RCW 18.08.410.

That said, the city still needs complete, accurate plans. Your site plan has to show property lines, setbacks and lot coverage correctly. Your floor plans have to meet the building code for things like egress windows, ceiling height and fire separation. Drawings that miss these come back with comments, and every round of comments adds time.

Some parts of the project do need a licensed professional. A geotechnical report must come from a licensed engineer. Retaining walls and unusual structural work, like cutting new openings into a foundation, usually need an engineer's stamp. If your lot is flat and the design is simple, you may never need one. If your lot is sloped, plan on it.

Watch for these

What Can Complicate Your Project

None of these usually kill a project. They change where the unit goes, what it costs, or how long it takes. The sooner you know which ones apply to your lot, the fewer surprises you get later.

Slopes and Soil: When You Need a Geotechnical Report

A geotechnical report is a soil and slope study done by a licensed engineer. It tells the city whether the ground can safely hold a new building. In Edmonds, the city's critical areas map is what triggers one. Critical areas are places the city treats as sensitive, like steep slopes, unstable soil, streams and wetlands. You can see what is mapped on your lot in the Edmonds Web Map.

Here is what to look for on that map:

• Steep ground. A strip of slope between 15 and 40 percent crossing your yard will likely lead the city to ask for a soil report. If the report rules out a spot, the unit moves to a spot that works.
• Earthquake soil. Much of Edmonds sits in a mapped seismic hazard area. Some lots are rated for liquefaction, which means loose, wet soil can act like liquid during an earthquake. Whether your rating counts as a regulated hazard is a call for a licensed engineer. We have found no exemption for ADUs.
• Landslide and erosion areas. If none are mapped near you and your lot is fairly flat, this usually does not apply. A lot that averages around 8 percent slope stays well under these limits.
• Streams and wetlands. These come with protective buffers, which are no build zones around them. A stream a few hundred feet away is usually too far to reach your lot.

The city's critical areas checklist makes the final call. The map is a guide, not a ruling. Edmonds does not publish how long a soil report takes or a list of approved engineering firms. If your lot needs one, plan for it as a real unknown and get schedules from local firms before you commit to a start date.

Sloped lots also bring two more permits. Edmonds requires a permit for grading, which is moving or leveling dirt, and a permit for any retaining wall.

Lot Coverage: The Math That Decides How Big You Can Go

Lot coverage is how much of your lot is covered by buildings. Edmonds caps it at 35 percent. That count includes your house, your garage, any sheds, and the new ADU. Here is how the math works.

In that example, a full size 1,200 square foot single story cottage fits exactly. A two story design would use less footprint and leave room to spare. A basement conversion adds little or no new footprint at all.

The catch is that county records are often fuzzy. They may not separate your basement from your main floor, or show the true footprint of your house. That gap can be the single biggest swing in how much room you really have. Records will not settle it. A real site plan or survey will.

There is a second number people mix up with lot coverage. It is called impervious surface, and it counts roofs plus driveways, walkways and patios. We found no separate cap on it in the Edmonds code. But it still matters, because it feeds into stormwater review below.

Fire Separation for Attached Units

Once part of your house becomes its own legal home, the two units need a fire rated wall and floor between them. A fire rated assembly is built with materials that slow a fire down, so a fire in one unit cannot spread fast into the other. For a basement ADU, that usually means upgrading the ceiling between the basement and the main floor. The exact rating needs to be confirmed with the city before design starts.

Egress and Basement Conversions

Egress means a safe way out in an emergency. Every bedroom in an ADU needs its own emergency escape window. In a basement, that often means cutting a larger opening into the foundation and digging a window well outside. This is real construction work, not paperwork.

Basement conversions have a few other checks too:

• Ceiling height. The space needs at least 6 feet 8 inches of ceiling height. Measure it before you plan anything else.
• A full kitchen. A kitchenette with a sink and a fridge does not count. A legal ADU kitchen needs a permanent cooking appliance, plus the gas or electrical work to run it.
• The separate entrance. A shallow basement makes a walkout or outdoor stairway easier. The design still has to be checked against your foundation.

Stormwater Thresholds

Stormwater is rain that runs off roofs and paving instead of soaking into the ground. Edmonds adds stormwater requirements once a project creates 2,000 square feet or more of new roof and paving combined. Past that line, you need a plan to manage that runoff on your lot.

Many ADUs add somewhere around 500 to 1,700 square feet of new hard surface, which stays under the line. A bigger unit with a new driveway or long walkway can cross it. A real site plan with the driveway and walkways drawn in is how you know for sure.

Other Items to Confirm

• Large trees. We have not found an Edmonds tree rule aimed at backyard ADUs outside a mapped hazard area. If a big tree sits near your build spot, an arborist confirms what applies.
• Short term rentals. The Edmonds ADU rules we reviewed say nothing about short stays. If you plan to rent by the night, confirm with the city before design.

Timeline

How Long Does an Edmonds ADU Permit Take?

Here is the honest answer. Edmonds does not publish a review time for ADU permits. Anyone who gives you an exact number before your design is set is guessing. What we can tell you is the order things happen in, and where the delays come from.

1. Lot review. Your property lines, hard surfaces, slopes and any alley get measured on the ground, not pulled from records.
2. Design. Unit sizes and locations get tested against the setbacks, the slope and your yard. You should see options before anything is drawn for the city.
3. Soil report, if required. If the critical areas checklist calls for one, a licensed engineer studies your site. Edmonds publishes no turnaround for this, so get real schedules from local firms before you commit to a date.
4. Permit submittal. The application goes in online through MyBuildingPermit.com with the site plan, floor plans, and any soil or stormwater documents.
5. City review. The city reviews the plans and sends comments. The design team answers them and resubmits as needed.
6. Permit issued. Once the city approves, construction can start.

The biggest swing factor is the soil report. A flat lot with no mapped hazards can skip it entirely. A sloped lot may wait on an engineer before design can be locked. Settling lot coverage and soil questions early is the best way to keep your timeline from slipping.